For me it's both sound and feel. I have used EJ44C (Extra Hard Tension) for decades and this on a 662mm scale instrument which used to have a very high (6mm/5mm) action that I dropped (5mm/4mm). It was not easy to play even with regular tension strings which was the reason I dropped the action in the first place. However, when I did this, I found that I could not play in my usual style which has a good attack and a relatively heavy stroke, without buzzes, particularly on the bass strings. So, I went to higher tension strings that suited my playing style and permitted good dynamic range with my instrument.
Yes, optimal string tension will vary from instrument to instrument in the sense that the top may be setup for a higher or lower tension strings, and "like" to be driven by a particular tension, but there are a number of other important factors to consider. These observations spring from my own personal experience and have a lot to do with my right hand nail configuration and technique.
My nails are small (think reduced contact area from the point of attack to the point of release) and rather sharply curved at the edges...more so on the inside of the finger. No matter how carefully I file, shape and smooth, I cannot get good tone playing in the traditional direction from the inside to the outside of the finger/nail. Years ago my teacher (who was an accomplished lutenist as well as a guitarist) suggested that I try playing from the outside towards the inside, as had been done by Ida Presti, Alexandre Lagoya and Alice Artzt. I tried it and immediately my tone production, power and security were significantly improved. My wrist is more elevated and curved/dropped to the outside and the curve of my fingers carries more from a vertical knuckle position above the strings. I have had no adverse effects with this hand position after more than 40 years.
For me, given my right hand position and nail shape, a higher tension string is better. Tone production and speed are improved in part due to the necessity of a slightly heavier attack and in part due to the quicker return of the string to a neutral position. This is a tactile factor which is also influenced by the action height and the break angle at the saddle.
I recently reduced the action on my guitar to around 4.5mm/3.5mm and began experimenting with different strings. This height is about as low as will work well with my Yamaha GC-10. Neck relief and frets are fine...in fact I recently used a fret leveling kit to correct some small irregularities at the 6th, 8th, 9th and 11th frets. Playability improved. I am now using Augustine Regal/Blue strings and am adapting my technique for a slightly lower tension "feel". The reason for this change is simple: my soon to arrive Yamaha GC82S is a 650mm scale guitar with a 5/32" (3.97mm) and 1/8" (3.18mm) action, and will come with the Augustine strings. I know the feel will be different (as the guitar is different...
), but it is most likely the feel with be lighter than I am used to at present. Who knows, I may decide that I want a higher action...but I will take some time before I make that decision.
I have also experimented with diamond string ties as I gradually decreased the tension "feel", and they do slightly increase the tension/resistance "feel" of the strings due to the increased break angle at the saddle.
All this is to say that when comparing different string tensions there are many factors to consider and my experience has been that I need 2 to 3 months to fully adjust to a different tension and then make an informed decision as to my preference.